This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/FI98/00244 which has an International filing date of Mar. 19, 1998 which designated the United States of America.
The present invention relates to a process for the preparation of precipitated calcium carbonate (PCC).
According to such a process calcium hydroxide is carbonated using gaseous carbon dioxide.
The invention also relates to an apparatus for carrying out the carbonation reaction and to a pigment product based on precipitated calcium carbonate.
Precipitated calcium carbonate is used as a paper filling or coating agent. Other possible fields of use include paints, plastics, the food processing industry, the pharmaceutical industry, etc.
PCC can be prepared by a causticizing process and by a carbonation reaction. In the causticizing process the calcium oxide is slaked whereby calcium hydroxide is formed which is then reacted with sodium carbonate in liquid phase. As a result, caustic soda (NaOH) and calcium carbonate are obtained, the sodium hydroxide remaining in dissolved state while the calcium carbonate is precipitated. Both products are recovered and forwarded to further processing.
In the carbonation process, the calcium hydroxide slurry obtained from slaking lime is reacted with gaseous carbon dioxide. This is typically carried out by charging a carbon-dioxide containing gas, derived from flue gas and having a CO2 content of about 20 to 40%, into the Ca(OH)2 mixture whose solids content is about 20%. The CO2 gas is hereby blown into an aqueous Ca(OH)2 solution whereby the gas is disintegrated into bubbles and the carbon dioxide contained in these bubbles is dissolved in the surrounding water. Carbonate ions are formed which react with the Ca2+ ions whereby calcium carbonate is obtained which is precipitated from the solution.
In order to provide an exhaustive account it may be mentioned that calcium hydroxide is reacted with carbon dioxide not only in the preparation of PCC but also in other contexts such as, among others, desulphuration of flue gases and scrubbing in a flue gas scrubber.
A number of considerable disadvantages are related to the prior art processes for preparing PCC. Thus, the conventional causticizing process is hampered by residual salts in the PCC. When, on the other hand, PCC is produced by conventional carbon dioxide carbonation processes, a disadvantage lies in the long carbonation time, typically 1 to 7 h, required by the reaction. In addition, the PCC crystals produces are of varying size and their particle size varies within a very wide range.
The prior art solutions have also been hampered by the difficulty of attaining sufficiently efficient mass transport conditions to enable fast nucleation and the simultaneous generation of a vast number of crystal seeds which would then grow into a vast number of small crystals.
The aim of the present invention is to remove the drawbacks of the prior art and to obtain an entirely novel solution for the preparation of precipitated calcium carbonate from slaked lime and carbon dioxide gas.
The invention is based on the concept of performing the carbonation subject to strong turbulence in a turbulence zone by reacting the carbon dioxide gas with calcium hydroxide particles by the intermediation of random liquid droplets. Thus, in the reaction, gas, liquid and solids particles are contacted with each other simultaneously under intense turbulence and a great energy intensity. The gas flow absorbs the liquid and the particles and forms a turbulent three-phase mixture. The solution according to the invention can also be termed a three-phase process because three phases are simultaneously present, the gaseous phase constituting the reaction medium.
The apparatus according to the invention comprises at least two serially arranged pin mills having one or more rotatable vane rings by means of which it is possible to subject the material charged into the apparatus to a great energy intensity. The first pin mill is furnished at least with an inlet for slaked lime and carbon dioxide and a discharge outlet for the reaction product and the second pin mill is furnished with an inlet for the product from the previous pin mill and a discharge outlet for the reaction product. Gas or blend liquor can, if desired, be fed between the rotating vane rings or groups of vane rings of the pin mills. The pin mills are connected to each other by means of pipes which can be furnished with inlets for blend liquors, if desired.
By means of the invention, completely novel products can be obtained having a shell-like structure. The precipitated particles preferably have a size of 30 to 100 nm and are of spherical shape whereby they are formed of one or several shell-like layers.
The invention offers considerable benefits. Thus, the carbonation of calcium hydroxide is extremely swift. The dwell time of the reaction may be as short as less than 1 second. Due to the great energy intensity carbonation may be performed at a high solids content (even at 40 to 60% by w.)
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The calcium carbonate obtained by the invention is of homogeneous quality; the diameters of the PCC produced may be e.g. 20 to 30 nm, 30 to 50 nm and 50 to 100 i.e. generally within the range from 20 to 100 nm, usually 30 to 100 nm. The minute PCC particles produced can be exploited in various ways: by combining them to form bigger pigment particles by means of van der Waals forces, particle clusters are obtained containing 10 to 30, typically about 15 to 20 joined particles. The formation of these particle clusters can be carried out by adjusting the pH to a value within the range from 6.2 to 10.8 whereby the Z potential of the particles is as small as possible. The particles can also be used for coating other pigments such as kaolin, chalk, talc, or titanium dioxide. The coating can be carried out by feeding the pigments to be coated e.g. in the form of an aqueous slurry together with calcium hydroxide and carbon dioxide into the apparatus of the invention and, if needed, by adjusting the pH value to a suitable range e.g. by introducing acid into the pin mill apparatus during production.
The carbonation is divided into several (e.g. 3 to 7) different process stages. The conversion of calcium carbonate increases step by step; depending on the dry matter content of the calcium carbonate it is usually close to 100 after 3 or 4 stages already. By dividing the process into stages blend components can be added to the different layers of the CaCO3 particle, said components affecting, among other things, the opacity and acidity resistance of the product. As an example a product may be cited prepared by a multi-step process wherein the obtained particles have a core layer consisting of calcium carbonate and a few shell layers consisting alternately of calcium phosphate and calcium carbonate, and a surface layer consisting of, e.g. calcium phosphate. Such a structure will improve the acid resistance of the calcium carbonate particles. In addition, variations in the refractive index between the different layers will provide improved opacity as compared to a mere CaCO3 particle.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: